The basic principle of high speed downlink packet access (HSDPA) is that the Node-B can make more efficient decisions and manage downlink radio resources on a short-term basis better than the radio network controller (RNC). In the meantime, the RNC still retains coarse overall control of a cell so that it can perform functions such as call admission control (CAC) and congestion control.
With the implementation of HSDPA, fast scheduling via layer 1 signaling becomes possible, which is a key advantage of HSDPA. In order to fully utilize this advantage, a well-designed scheduling algorithm for HSDPA is required.
One scheduling algorithm that has been proposed for HSDPA is a maximum C/I (carrier to interference ratio) algorithm which always schedules the WTRU with the highest channel quality, (i.e., C/I). Although this algorithm takes advantage of instantaneous channel quality, it does not consider fairness between WTRU users.
Another scheduling algorithm that has been proposed for HSDPA is a round robin algorithm disclosed in a publication entitled “Network Performance of Transmit and Receive Antenna Diversity in HSDPA under Different Packet Scheduling Strategies,” by J. Ramiro et al., VTC 2001, where each WTRU uses the same resources. Although fairness is achieved between WTRU users, this algorithm totally ignores channel quality in scheduling.
Yet another scheduling algorithm that has been proposed for HSDPA is a proportionally fair algorithm which schedules the WTRU with the highest normalized C/I, as disclosed in a publication entitled “Interaction of Transmit Diversity and Proportional Fair Scheduling,” by L. Berger et al., VTC Fall 2002, and in another publication entitled “Design of Packet Transmission Scheduler for High Speed Downlink Packet Access System,” by W. S. Jeon et al., VTC Spring 2002.
The normalized C/I of the WTRU is the C/I normalized by the WTRU's average throughput within a specific time window. This algorithm is a promising solution to the scheduling of HSDPA. It balances between instantaneous channel quality and overall fairness among WTRU users. However, it has at least three deficiencies.
The first deficiency is that only one WTRU with the best normalized channel quality is scheduled at a time, which may not have enough data to utilize all the physical resources reserved for HSDPA.
The second deficiency is that a WTRU's average throughput within a specific time window does not consider that different WTRUs may have different data rates, and thus it is not fair for WTRU users with higher data rates.
The third deficiency is that the bursty nature of data traffic is not considered. The average throughput during inactive time, (e.g., reading time), is meaningless. A WTRU that just transitions from an inactive state (reading time) into an active state (data bursts) will get unjustified preference in scheduling.